CH. IIT. DOWNWARD GROWTH OF THE ROOTS. 



105 



Tliat liglit, not gravity, is tlie main conductor of 

 tlie growth of tlae heads of phuits is probable from 

 the fact that, where trees stand close together, their 

 chief growth is upwards, and their side-branches die ; 

 and as long as their stems are thus in the shade, they 

 show no disposition to shoot out sideways again. But 

 the moment such an over thick wood is over-thinned, 

 the stems burst out sideways to the light which is 

 admitted. And he who is most wedded to the extra- 

 ordinary paradox that the leader owes its vertical 

 direction to gravity, will, I think, scarcely assert that 

 the same cause produces the horizontal growth of the 

 branch. 



Paradoxical as it may sound, if a side-branch of a 

 tree descends from a height till it touches the ground, 

 its growth all the time it is descending is rather upward 

 than downward ; that is, the new growth, or shoot, at 

 the end of such a bough is generally slightly curved 

 upwards by the action of hght on the cellular structure 

 of its upper side. Gravity, indeed, draws the whole 

 branch down bodily, for light has no power to act 

 through the dead bark ; but light will so draw the new 

 end up against gravity, that, when the branch comes 

 to the ground, it will rest on a curved elbow, not on 

 its end. This fight between gravity and hght is the 

 origin of very beautiful growth in many trees. 



Were I to lay down a general rule about the direc- General 

 tion of the growth of the greater part of plants, it head^o- 



wards 



would be, that the growth above is in whatever ^^s^^J 



